A novel imidazophenazine-based metallogel act as reversible H2PO4− sensor and rewritable fluorescent display material

Sensors and Actuators B 251 (2017) 250–255

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A novel imidazophenazine-based metallogel act as reversible H2 PO4 − sensor and rewritable fluorescent display material Qi Lin a,∗ , Feng Zheng a , Tao-Tao Lu a , Juan Liu b,∗ , Hui Li a , Tai-Bao Wei a , Hong Yao a , You-Ming Zhang a a Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China b College of Chemical Engineering, Northwest University for Nationalities, Lanzhou 730000, PR China

a r t i c l e

i n f o

Article history: Received 12 September 2016 Received in revised form 4 May 2017 Accepted 11 May 2017 Available online 12 May 2017 Keywords: Chemosensor Metallogel Organogel Imidazophenazine Fluorescent display material

a b s t r a c t By rationally introduced 1H-imidazo[4,5-b]phenazine moiety into the gelator molecular, a novel organogelator G5 was designed and synthesized. Imidazophenazine moiety could act as fluorescent group as well as binding site. The gelator G5 could form a stable organogel (OG5) accompanied with dramatic aggregation-induced emission enhancement (AIEE) in DMF. The AIEE of OG5 could be quenched by adding Fe3+ into the OG5. Meanwhile, OG5 could form no fluorescence stable metallogel (FeG) with Fe3+ . Interestingly, the metallogel (FeG) could reversibly fluorescent “turn-on” sense H2 PO4 − with high selectivity and sensitivity. The detection limit of FeG for H2 PO4 − is 1.0 × 10−6 mol L−1 , moreover, other ions did not interfere in the sensing process. The H2 PO4 − sensing mechanism was confirmed based on competitive coordination between gelator, Fe3+ and H2 PO4 − . The metallogel FeG could act as reversible H2 PO4 − sensor and convenient H2 PO4 − test kits. In addition, the FeG film also could act as an rewritable fluorescent display material. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent years, stimuli-responsive supramolecular gels [1–8] have attracted increasing attention due to their promising applications in sensors, biomaterials, surface science, displays, etc. [9–15]. Generally, supramolecular organogels are formed by assembling low molecular-weight gelators (LMWGs) into physically cross linked three-dimensional networks with solvent molecules entrapped inside through noncovalent intermolecular interactions, such as hydrogen bonding, ␲–␲ stacking, van der Waals (vdW), electrostatic interactions, and so on [16–22]. By taking advantage of the dynamic and reversible nature of noncovalent interactions, the stimuli-responsive supramolecular gels can sense, process, and actuate a response to an external change without assistance [16–24]. Moreover, recently, the metallogel received more and more attentions because the tuneable coordination binding strength, as well as the fascinating redox, optical, electronic, or magnetic properties of the metal ions would benefit the application of metallogel-based stimuli-responsive supramolecular gels

∗ Corresponding authors. E-mail addresses: [email protected] (Q. Lin), [email protected] (J. Liu). http://dx.doi.org/10.1016/j.snb.2017.05.053 0925-4005/© 2017 Elsevier B.V. All rights reserved.

in materials science [25–29]. However, it is still a big challenge to design and synthesize novel stimuli-responsive supramolecular gels that can optically sense a given chemical stimulus with specific selectivity. Therefore, novel and efficient fluorescent groups are needed to improve the gelator as well as supramolecular gels’ optically properties. What is more, H2 PO4 − is one of the important elements of constituting genes and genetic materials in life systems. It also plays an important role in information transfer and energy storage [33–35]. Much interest has been sparked in the design of new tactics to monitor H2 PO4 − in biological and environmental samples. Recently, various fluorescent chemosensors for H2 PO4 − of high selectivity and low background disturbance have been reported [36–38]. Although the supramolecular gel-based ions sensor shown the merits of efficient and convenient to use, however, the supramolecular gel-based fluorescence sensor for H2 PO4 − is rarely reported. Moreover, as a very useful material, security display materials [39,40] are composed of invisible substances that provide printed images that are not able to be photocopied, and are readable only under special environments. Although thermally rewritable printing media have already been developed, security display materials that allow rewriting of invisible printed images are very rare,

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[39,40] especially, supramolecular metallogel-based fluorescent display materials were less reported. In view of these, and as a part of our research in ionsresponsive supramolecular gels, [24,29–32] herein, we report a novel imidazophenazine-based supramolecular metallogel (FeG), which could act as H2 PO4 − sensor as well as a rewritable fluorescent display material. The design strategies for the metallogel are as follows. Firstly, we rationally introduced 1Himidazo[4,5-b]phenazine moiety into the gelator G5 molecular, the imidazophenazine moiety could act as fluorescent group, binding site and ␲–␲ stacking site. Secondly, multi self-assembly driving forces such as strong van der Waals (vdW) existing in the long alkyl chains and ␲–␲ stacking existing in imidazophenazine groups have been introduced into to the gelator G5 to provide better gelation abilities. Finally, in order to obtain an ions responsive supramolecular gel, competitive coordination between metal ions in metallogel and anions have been introduced into the gels. As we expected, the gelator G5 could form stable supramolecular organogel OG5 in DMF with strong aggregation induced emission enhancement (AIEE) [41–49] at very low critical gelation concentrations (CGCs). Upon addition of Fe3+ into OG5, a stable Fe3+ -coordinated supramolecular metallogel (FeG) with no fluorescence could be formed. The FeG could reversibly sense H2 PO4 − with high selectivity and sensitivity. The FeG-based ions responsive film has been prepared and which could act as a novel rewritable fluorescent display material. 2. Experiment 2.1. Materials All reagents and starting materials were obtained from commercial suppliers and used as received unless otherwise noted. All anions were used as sodium or potassium salts while all cations were used as the perchlorate salts, which were purchased from Alfa Aesar and used as received. Fresh double distilled water was used throughout the experiment. 2.2. Instruments Nuclear magnetic resonance (NMR) spectra were recorded on Varian Mercury 400 and Varian Inova 600 instruments. Mass spectra were recorded on a Bruker Esquire 6000 MS instrument. The X-ray diffraction analysis (XRD) was performed in a transmission mode with a Rigaku RINT2000 diffractometer equipped with ˚ The morgraphite monochromated CuKa radiation ( = 1.54073 A). phologies and sizes of the xerogels were characterized using field emission scanning electron microscopy (FE-SEM, JSM-6701F) at an accelerating voltage of 8 kV. The infrared spectra were performed on a Digilab FTS-3000 Fourier transform-infrared spectrophotometer. Melting points were measured on an X-4 digital melting-point apparatus (uncorrected). Fluorescence spectra were recorded on a Shimadzu RF-5301PC spectrofluorophotometer. 2.3. Synthesis of G5 2,3-Diamino-phenazine [50] and 2,3,4-tris-hexadecyloxybenzaldehyde [29] were prepared according to the reported procedure. 2,3,4-Tris-hexadecyloxy-benzaldehyde (1 mmol), 2,3-diaminophenazine (1 mmol) and p-toluenesulfonic acid (0.05 mmol, as a catalyst) were added to DMF (15 mL). Then the reaction mixture was stirred at 80 ◦ C for 24 h, after removing the solvent, yielding the precipitate of G5. The synthesis process of gelator G5 is demonstrated in Scheme 1 (yield: 76%). m.p. 90–92 ◦ C. 1 H NMR (Fig. S1) (CDCl , 400 MHz): ı, 10.91 (s, 1H, ArH), 8.61 3 (s, 1H, ArH), 8.33–8.20 (m, 4H, ArH), 7.78–7.77 (m, 2H, ArH), 6.87–6.86 (d, J = 4.0 Hz, 1H, ArH), 5.02–4.98 (d, J = 16.0 Hz, 1H,

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Fig. 1. Fluorescence spectra and photograph of organogel OG5 and sol (in DMF, 1.0%, ex = 475 nm).

NH), 4.34–4.05 (m, 6H, OCH2 ), 1.91–1.83 (m, 6H, OCH2 CH2 ), 1.53–1.52 (d, J = 4.0 Hz, 6H, CH2 CH3 ), 1.26–1.21 (m, 72H, CH2 ), 0.89–0.87 (t, J = 8.0 Hz, 9H, CH3 ). 13 C NMR (Fig. S2) (CDCl3 , 100 MHz) ı/ppm 158.12, 156.89, 152.44, 148.04, 142.45, 142.43, 141.44, 140.90, 140.83, 139.09, 129.42, 129.18, 129.06, 125.92, 115.39, 114.11, 112.79, 111.78, 109.08, 105.38, 75.32, 73.97, 68.97, 31.92, 31.91, 30.47, 30.35, 29.75, 29.73, 29.71, 29.67, 29.64, 29.61, 29.59, 29.58, 29.47, 29.39, 29.36, 29.35, 29.22, 26.18, 26.09, 22.68, 14.10. IR (KBr, cm−1 ) v: 3431 (N H), 1686 (C N); MS-ESI calcd for C67 H109 N4 O3 [G5+H]+ : 1017.8494; found: 1017.8246 (Fig. S3). 3. Results and discussion At the beginning, we investigated the self-assemble properties of G5 in various solvents by means of the “stable to inversion of a test tube” method (Table S1). As a result, the multi-selfassemble driving forces in G5 provided the gelator with better gelation ability, G5 showed excellent gelation ability in isopropanol, petroleum ether, n-propanol, DMSO, ethanol and DMF simultaneously. Among these solvents, OG5 in DMF showed lower CGC (0.2%, w/v, 10 mg/mL = 1%) and higher gel–sol transition temperature Tgel (Fig. S4 and Table S1). Furthermore, the sol–gel state can change rapidly in DMF solution. After heating the DMF solution of G5, the stable gel OG5 was gained within 1 min. This gel was found to be stable in closed test tube for at least five weeks at room temperature. More interestingly, as shown in Figs. 1 and 2, G5 has weak fluorescence emission band at 554 nm in hot DMF solution (T > Tgel ). However, when the sol changed to the organogel OG5 (T < Tgel ), the OG5 shows strong fluorescence emission band at 614 nm, which indicated that the fluorescence of OG5 can be attributed to the aggregation-induced emission enhancement (AIEE). As shown in Fig. 2, the aggregation process was monitored by temperature dependent fluorescence spectra. When the temperature of the sol dropped below Tgel of OG5, the emission intensity at 575 nm suddenly increased and reached a steady state in about 1 min by AIEE. Then, we carefully investigated the metal ions response properties of the OG5 (1.0%, DMF) by adding various metal ions (including Mg2+ , Ca2+ , Cr3+ , Fe3+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Ag+ , Cd2+ , Hg2+ , Pb2+ , Ba2+ , Sr2+ , and Al3+ ) into the OG5. The addition and diffusion of various metal ions to the supramolecular organogel OG5 could generate the corresponding metallogels (named as MGs, for example FeG, MgG, CaG, CrG and so on) respectively (Fig. S5a). Meanwhile, the addition of metal ions could induce the AIEE of OG5 take place obvi-

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Scheme 1. The synthesis of G5.

Fig. 2. Temperature-dependent fluorescent spectra of OG5 (in DMF, 1.0%) during gelation process (ex = 475 nm).

Fig. 3. Fluorescence spectra of OG5 (1.0%, in DMF) and OG5 in the presence of Fe3+ at room temperature (ex = 475 nm).

ous changes (Fig. S5b). Interestingly, as shown in Fig. 3, upon the addition of 1 equiv of Fe3+ into the OG5, the AIEE of OG5 has been quenched and formed corresponding no fluorescence metallogel FeG. Then, we introduced competitive binding interaction between metal ions, anions and gelators by adding various anions into the metallogel FeG. As shown in Fig. 4, with the addition of two equivalents of water solutions of various anions (F− , Cl− , Br− , I− , AcO− , H2 PO4 − , HSO4 − , N3 − , SCN− , S2− , ClO4 − , and CN− ) into the metallogel FeG, respectively, only H2 PO4 − could induce the metallogel FeG emitting the strong fluorescence at 570 nm immediately. While other anions such as F− , Cl− , Br− , I− , AcO− , H2 PO4 − , N3 − , CN− , SCN− , ClO4 − and S2− could not induce any significant emission changes. Therefore, the FeG could selectively sense H2 PO4 − in water solution. Competitive experiments (Fig. S6) also indicated that other metal ions (Ca2+ , Ba2+ , Ca2+ , Cd2+ , Pb2+ , Ni2+ , Cr3+ , Mg2+ , Sr2+ , La3+ , Co3+ , Cu2+ , Al3+ , 1 × 10−2 mol L−1 ) could not induce any significant

Fig. 4. Fluorescence spectra of OG5 (1.0%, in DMF) in the presence of various anions (F− , Cl− , Br− , I− , AcO− , H2 PO4 − , HSO4 − , ClO4 − , N3 − , SCN− , S2− and CN− , respectively, using their 0.1 mol L−1 sodium or potassium salts water solution as the sources) at room temperature (ex = 475 nm).

Fig. 5. Fluorescence spectra of FeG (1.0%, in DMF) with increasing concentration of H2 PO4 − (using 0.1 mol L−1 NaH2 PO4 water solution as the H2 PO4 − sources), ex = 475 nm.

interfere into the sensing process of FeG for H2 PO4 − . Moreover, the pH dependence of the FeG for H2 PO4 − was also examined. The solutions of H2 PO4 − in water at pH values ranging from 1 to 12 were added to the FeG respectively. The results indicated that the suitable pH range for FeG detection H2 PO4 − is pH 1.0–7.0. (Fig. S7). The detect limit of the FeG for H2 PO4 − is also investigated. When gradually adding H2 PO4 − into metallogel FeG, the emission intensity at 570 nm increased with the increasing concentration of H2 PO4 − (Fig. 5). The detection limit of the fluorescence spectra changes calculated on the basis of 3sB /S was 1.0 × 10−6 M (1 ␮M) for H2 PO4 − anion [51]. To investigate the sensing mechanism, the FT-IR and X-ray diffraction studies were carried out. In the FT-IR (Fig. S8), the C N vibration absorption of OG5 appeared at 1680 cm−1 . However, when adding 1 equiv of Fe3+ to the OG5, the C N vibration absorp-

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Scheme 2. Chemical structure of the G5 and the presumed self-assembly and reversible stimuli-response mechanism.

tion of xerogel FeG moved to 1651 cm−1 . Upon addition H2 PO4 − , the C N vibration absorption appeared at 1672 cm−1 again. What is more, the stretching vibrations of NH of OG5 xerogel shifted to high wavenumbers when G5 interacted with 1 equiv of Fe3+ . After adding 2 equiv of H2 PO4 − to the FeG, the stretching vibrations of NH of G5 recovered the lower wavenumbers. These results revealed that G5 coordinated with the Fe3+ . After adding H2 PO4 −

to the FeG, the Fe3+ competitively coordinated with H2 PO4 − , then, the gel state of OG5 restored (Scheme 2). Moreover, X-ray diffraction studies of xerogels were carried out to obtain the structural insight into the gelator molecules and in the assembled gel state (Fig. S9). The OG5 showed peaks at 2 = 19.88, 21.70, 22.70 corresponding to the d spacing’s of 4.46, 4.11, 3.90, which suggested the self-assemble of the supramolecular organogel. However, these peaks disappeared when adding

Fig. 6. SEM images of (1) the powder of G5; the xerogel of (2) OG5; (3) FeG and (4) FeG xerogel treated with H2 PO4 − in situ.

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act as not only a convenient reversible H2 PO4 − detection test kit, but also an erasable dual-channel fluorescent display material. 4. Conclusion A novel phenazine derivative gelator G5 was designed and synthesized and a supramolecular metallogel FeG was prepared by the competitive coordination of Fe3+ with the OG5. The results proved that the FeG could sense H2 PO4 − with high selectivity and sensitivity in aqueous solution. The detection limit of the fluorescence spectral change is calculated on the basis of 1.0 × 10−6 M. Moreover, H2 PO4 − -responsive film based on FeG has been developed. The FeG film could act as not only a convenient reversible H2 PO4 − detection test kit, but also an erasable fluorescent display material. Fig. 7. Fluorescent “OFF–ON–OFF” cycles of FeG controlled by the alternative addition of Fe3+ and H2 PO4 − , ex = 475 nm.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (Nos. 21574104; 21662031; 21661028), the Natural Science Foundation of Gansu Province (1506RJZA273) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT 15R56).

Fig. 8. Writing and erasing of a natural light invisible image on FeG-based film. The photographs were taken at room temperature under nature light (N) and exposure to a 365 nm ultraviolet light (UV).

1 equiv of Fe3+ to the OG5. Interestingly, after adding 2 equiv of H2 PO4 − to the FeG, the strong peak at 2 = 19.88, 21.70, 22.70 appeared again, which confirmed that the H2 PO4 − competitively bound to Fe3+ and the gel recovered again (Scheme 2). The morphologies of powder of G5 and xerogels were investigated by SEM (Fig. 6). The image demonstrated that the powder of G5 is about 100 nm in thickness which is disordered. And the organogelator molecules in the gel phase were self-assembled into film with holes structures, which further cross-linked into 3-D networks. However, the SEM images of metallogel FeG showed an overlapped rugate layer structures. Nevertheless, adding H2 PO4 − into the FeG, the image showed film with holes structures again. These phenomena indicated that the gelator G5 coordinated with Fe3+ , significant changes took place on the self-assembly states. Meanwhile, after treating the FeG with H2 PO4 − water solution, the micro-structure of FeG changed to OG5. These indicated that the H2 PO4 − competitively bound to Fe3+ and OG5 recovered again (Scheme 2). Moreover, the reversibility of the H2 PO4 − sensing process also supports above proposed mechanism. For example, the metallogel FeG could selectively fluorescent “turn-on” sense H2 PO4 − , while, upon the addition of Fe3+ into the H2 PO4 − -contained FeG, the fluorescence of FeG quenched, which was attributed to the Fe3+ coordination with gelator again. These properties make FeG act as a H2 PO4 − and Fe3+ controlled “OFF–ON” fluorescent switch. By alternating addition of H2 PO4 − and Fe3+ , the switch could be reversibly performed at least for three cycles with little fluorescent efficiency loss (Fig. 7). In order to facilitate the use of the FeG, the rewritable fluorescent display material bearing on FeG film was prepared by pouring the heated DMF solution of FeG onto the clean glass surface and then drying in the air. As shown in Fig. 8, the FeG film has no fluorescence emission under 365 nm ultraviolet light, however, when writing on the film with a writing brush dipped in H2 PO4 − water solution, the brilliant orange fluorescent of the writing image appeared. Interestingly, the fluorescent writing image could be erased by brushing in Fe3+ on the corresponding films again. Therefore, the FeG film could

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Biographies Qi Lin received the master’s degree from Department of Chemistry at Northwest Normal University in 2005. In the same year, he joined the Department of Chemistry at Northwest Normal University. He obtained PhD. in 2009 and became an associate professor in the same year. His research interests include supramolecular chemistry and supramolecular functional materials. Feng Zheng is a graduate student in Northwest Normal University. His major is inorganic chemistry. Tao-Tao Lu is a graduate student in Northwest Normal University. Her major is inorganic chemistry. Juan Liu is an associate professor of College of Chemical Engineering, Northwest University for Nationalities. Her research interests include supramolecular chemistry and supramolecular functional materials. Hui Li is a graduate student in Northwest Normal University. His major is inorganic chemistry. Tai-Bao Wei received the master’s degree from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences in 1991. In the same year, he joined the Department of Chemistry at Northwest Normal University. He became professor in 2000. His research interests include synthetic chemistry and supramolecular chemistry. Hong Yao is a an associate professor of Department of Chemistry, Northwest Normal University, Her research interests include supramolecular chemistry and inorganic chemistry. You-Ming Zhang received the master’s degree from Qinhai Institute of Salt Lakes, Chinese Academy of Sciences in 1988. In the same year, he joined the Department of Chemistry at Northwest Normal University. He became professor in 1999. His research interests include supramolecular chemistry and Coordination Chemistry.